Capturing CO2 from ambient air - David Keith

(Rao and Rubin, 2002; Rao, 2004).
The capture of CO2 from calcination might alternatively be achieved using oxygen blown combustion
in a fluidized bed. Such a system would be a hybrid of existing fluid bed calciners and oxygen-fired coal
combustion with CO2 capture which has been studied (but not implemented) as a method for achieving
CO2 capture at electric power plants. Such a system might offer significant energy savings over the amine
system but because it introduces significantly more new engineering and so we do not include it in our
base example system. Other authors estimating the energy requirements of an example air capture system
have assumed an oxyfuel system (Baciocchi et al., 2006; Zeman, 2006).
2.3.4 Integrated system
We argue that one may construct an air capture system essentially by stringing together the components
described: a contactor, a kraft recovery system, an amine-based capture system, and a compressor. A
few novel interconnections are required, like a heat exchanger between the contactor and the causticizer
and heat exchange between the calciner flue gas and amine recovery boiler. We expect the capital and
maintenance for this and some extra piping will be more than balanced by the capital equipment not
required by an air capture system, compared with the caustic recovery system for a paper mill. Thus for
the caustic recovery portion, we use a rule-of-thumb capital cost and energy requirements estimate for a
“turn-key” caustic recovery system, obtained from an industry engineer (Flanagan, 2004). The capacity of
this plant is 1000 ton-CaO per day, or about 300,000 ton-CO2/yr. This is comparable to the throughput of
a single large contacting tower as detailed in the next chapter. However, a full-scale air capture installation
would likely include dozens or hundreds of such towers, allowing for a combined caustic recovery plant
one or two orders of magnitude larger in scale. There may be significant savings of capital and maintenance
cost at this larger scale, as well as opportunities for more energy-efficient operation. These benefits of
economy-of-scale are not considered in our example system estimates.
The parameters for the amine capture plant are derived from the model used in Rao and Rubin (2002)
with adjustments made for the higher CO2 concentration in calciner flue gas. Compression is included in
the amine plant. The amine system is assumed 90% efficient, so that all upstream values must be adjusted
up to compensate for CO2 leakage.
2.3.5 Energy requirements
Table 2.4 shows energy requirements of various components of the example system, along with comparable
values from the analysis of Baciocchi et al. (2006) and Zeman (2006). They both assume more
optimized systems than the pulp and paper mill caustic recovery system used in our example system.
Baciocchi et al. have two cases; in case A they assume a conventional dewatering system and in case B
they assume a newer pellet reactor system which removes water much more efficiently. This turns out to
have a substantial effect, as the reduced demand for calciner heat multiplies through other components,
reducing demand for oxygen and compression.
Another important factor determining total energy requirements is the handling of low-grade heat from
the slaking reaction and from steam in the flue gas. Neither of these are typically utilized in a kraft mill.
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